GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



411 



throw near the river, but it increases rapidly northward, Utica 

 shale on the east side adjoining Precambric rock on the other, 

 so that the throw there would seem of equal magnitude with that 

 of the Little Falls fault. The Noses fault involves the entire 

 thickness of the Beekmantown and Trentom formations, the latter 

 only 17 feet thick here, together with an unknown amount of the 

 Utica. The two former aggregate 500 feet, so that a minimum 

 value is thus given, "to which must be added the thickness of Utica 

 involved. According to Prosser the Hoffman fault throws out the 

 entire Utica and Trenton and some of the Beekmantown, so that 

 its throw is just about 1600 feet.^ It is, then, the greatest of the 

 Mohawk faults, as it is also the most easterly. 





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Pig. 5 To illustrate extension owing to normal faulting. First, the unfaulted 

 block is shown with the position of the fault plane, second, the block after faulting, 

 with the original position dotted. The lateral extension, or heave, is manifest. The 

 hade of the fault is 15° and the lateral extension about one fourth of the vertical 

 displacement. 



This whole faulted district closely adjoins on the west the 

 district of Appalachian folding. This is a region of sharply com- 

 pressed rocks, producing folding and thrust faulting. The region 

 under consideration was but slightly affected by these forces, 

 thrust faults being absent and folds present only in the most 

 minor degree. Normal faults are however abundant, in fact are 

 present in both districts. But normal faulting implies surface 

 tension instead of compression, since, except in cases where the 

 hade is absolutely vertical, the rocks have greater lateral extent 

 after the faulting than before [fig. 5]. Since absolutely vertical 

 faults are exceptional, much normal faulting in a district of 

 parallel faults must produce a respectable amount of surface ex- 

 tension. The period of tension would seem to have followed that 



'N. Y. State Mus. Bui. 34, p.476. 



